This proposal is designed to determine whether the levels of iron supplied in the diets of rats has any influence on the susceptibility of these animals to pyelonephritis produced experimentally with Proteus mirabilis as the infectious agent. Experimental rats will be maintained on diets containing amounts of iron designated to achieve various levels of iron deficiency. Control rats will receive sufficient iron in their diets so that they will exhibit no signs of iron deficiency whatsoever. The degree of iron deficiency will be monitored so that an indication can be obtained as to whether the maintenance of this nutritional state has any effect on the host other than as it relates to pyelonephritis. In addition, the stage in the development of pyelonephritis at which iron exerts its effect will be investigated by studies involving the effect of iron on colonization of the renal pelvis by P. mirabilis as well as its effect on the bactericidal action of polymorphonuclear leukocytes.